Stationary Engine List

Making Mufflers

Helen French

| March/April 2002

To begin with, another update to the 'Tillie' story.
She finally, and with some difficulty, made it into our garden,
much to the bemusement of our neighbors, who have been asking which
direction we intend to fire our 'cannon.' She has indeed
fired, and run for some length of time. On Hew Year's Day, as
part of the annual Worldwide Engine Crankup, she ran alongside a
selection of smaller English and American engines - despite
below-freezing temperatures resulting in a very happy
owner!

And so on to the discussion that attracted my attention this
month on the ATIS Stationary Engine Mailing List. As ever, the
following comments reflect a variety of opinions that surfaced
during this discussion.

I'm thinking about building my own muffler for my Galloway.
My plan is to use a NPT flange. Has anyone built one using this
idea? What goes inside the muffler, i.e., baffles? I haven't
figured out the 'top dome' part yet, but possibly someone
out there can help? They're a little pricey and it would be fun
to build my own.

For a domed top, why not take a piece of flat plate, heat it up
cherry red, and hammer it to a domed shape with a flat rim? Bolt it
to your flange with a flat washer or two as a spacer? You might
also be able to find some assorted cast iron bits (floor trap,
etc.,) at a plumbing supply house that would have a domed shape.
You'd probably need to weld some bits to it, but it would be a
start. Have fun, get creative. And you don't need anything
inside - the spacing between the two halves controls the amount of
bark.

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Someone mentioned using the bottom of a fire extinguisher for
the dome, they're usually available at extinguisher service
centers as rejects.

That was me! I attempted to fashion an authentic-looking
replacement for the muffler on my 10 HP International M. As for
using old fire extinguishers, the old soda acid-type water
extinguishers are generally a canister of thin brass, and even the
more modern water-stored pressure types are fairly thin steel. I
used an old CO2 extinguisher for my dome and found it to
be of surprisingly heavy gauge metal. I'd recommend using the
CO2 extinguishers because they are designed to hold a
fair amount of pressure.

I'd looked at floor flanges with the idea of building a
muffler, several years back. A variation that I came up with (but
haven't built) is to bolt two floor flanges together with
washers between, the circumferential slit being the outlet.
Opposite the inlet pipe is another pipe capped at the opposite end.
It serves as a resonator and is a well-known type of silencer in
acoustics.

If the pipe is of the proper length to reflect the main
frequency of the sound back 180 degrees out of phase at the outlet
slit, this 'branch muffler' can be made almost 100 percent
silent. To avoid the cumbersome length of the pipe, a Helmholtz
resonator could be used instead, with a small pipe running into a
large hollow chamber.

I looked at grease traps for this, also pipe expanders for
larger diameter, capped resonating pipes. I thought about drilling
and welding a smaller pipe into a cap for a much bigger pipe, or
something like a small gas tank or fire extinguisher casing. This
offers some additional expansion volume and would reduce the back
pressure offered by the floor flange simply having a baffle plate
bolted up against it with a small exhaust slit. It also might allow
use of a wider exit slit without producing as much noise, and
dropping the pressure would tend to reduce the annoying hiss that
these types of mufflers tend to make from gas escaping at the
exhaust's sound speed (supersonic relative to the cool ambient
air) when the pressure gradient across the opening is
'critical,' or around one atmosphere or higher.

Supersonic gas jets produce shock waves and turbulence that, on
this scale, ends up mostly producing lots of really high
frequencies, much of the acoustic energy being ultrasonic.

The reproduction cone muffler I have been using on my
Indian-built Lister clone works this way, by letting the gas escape
through a 1-mm high slot around the circumference of a 4-1/4-inch
circle (giving a total area similar to the cross-section of a
one-inch pipe) and then turning the resulting sheet of supersonic
gas upward with a pressed flange to basically roll it into a
cylinder with shear turbulence on both the inside and outside,
rapidly slowing the gas. It works well enough, but I've grown
weary of the hissing tone. Fairbanks-Morse mufflers and IHC LA/LB
mufflers are similar, as are many others.

The 'branch muffler' principle with a Helmholtz
resonator is widely used in automotive mufflers, usually as a
hollow chamber at the rear of the muffler shell with no outlet, the
inlet being a short piece of pipe lined up with (but not in contact
with) the muffler's main inlet pipe in the first reversing
chamber right ahead of the resonator.

Choice of the resonant frequency is critical. If wrongly
selected it can really make the exhaust boom, but if done right the
sound waves cancel out at the point where they come out. Really, a
classic branch muffler would tee out straight to the side of the
pipe line, sound waves going out into it and coming back into the
line delayed from the initial impulse so it interferes
destructively. Here's another thought; just use a pipe tee (or
'Y') with two outlet pipes of suitably chosen lengths, one
capped at the end and one open.

There are basically only three ways to silence exhaust noise:
Reflection, restriction and absorption. How you use and combine
these is limited only by your imagination.

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I read in one of my old books that a muffler should be 12 times
the displacement of one cylinder. I don't think that anything
needs to go inside the chamber. As I understand things, the goal is
to reduce the exhaust gas velocity when it enters OUR space. I
suspect the 12 times chamber does this so that when the gas exits
the chamber the pressure is reduced as well as the velocity.

Many old books recommended three to five times the cylinder
displacement as a suitable volume for a muffler of the expansion
chamber type. That'd drop the pressure at the outlet below the
critical level, so no supersonic gas comes out of the stack. With
low-speed engines (having slowly opening exhaust valves) it'd
be quiet enough for most purposes.

Last weekend I made a muffler for a B&S 3 HP engine in my
Bantam tractor. The engine has a 3/4-inch NPT exhaust port opening.
The muffler I built looks like one that would be on a big tractor.
It has 3/4-inch pipe at the bottom end, extending into the main
chamber about four inches and has 96 3/32-inch holes drilled in
this section. A cap was welded to the inner end of this pipe so the
hot exhaust gas would have to go through the small holes. The main
body of the muffler is made of a 12-inch piece of two-inch diameter
thin-wall tubing. A washer-type ring was made and welded to the
main body end and to the 3/4-inch inlet pipe. On the outlet end, I
made another washer ring to fit the inside of the two-inch tubing
and a hole in it to fit a piece of 3/4 inch EMT thin-wall conduit.
This was left open at both ends and welded to the ring and body of
the unit.

I can't believe the difference this made over just a piece
of pipe - a nice mellow sound out of a Briggs and Stratton. Hard to
believe, but true. I didn't use any type of packing inside.

I know of a 1907 Otto gasoline model that is the quietest
I've ever heard. Its muffler is a big cast iron pot about the
size of a propane bottle for a barbeque grill, with the inlet in
the side and threads for an outlet pipe in the top center
that's much smaller than the inlet. I'd say it's a
3/4-inch or one-inch water pipe. He uses a piece about four-foot
long as the stack. If you want a quiet muffler, that's a good
plan to follow. Use a big inlet and lots of expansion volume to
keep the backpressure down, but go small on the outlet. There's
lots of time for the gas unloaded into the expansion chamber at the
beginning of the exhaust stroke to escape to the atmosphere with
our low speed engines. It doesn't have to blast out explosively
to allow the engine to breathe reasonably well.

Most guys use a big outlet to allow for gas expansion, but that
allows the sound to escape from the outlet more readily than to
reflect back toward the engine. Think of the little pipes used by
high-speed engines like Briggs & Stratton of similar
horsepower. The average gas-flow rate is about the same with the
big, slow engine, and one of those little outlets will work fine if
a big expansion chamber soaks up the large volume pulse at exhaust
valve opening.

My suggestion is, use the biggest expansion chamber you can
conveniently rig up, at least five times the cylinder displacement
volume, use an inlet pipe of the same size as your engine's
standard exhaust, and an outlet pipe of the size used by typical,
modern, small engines of equivalent horsepower, e.g., 1/2-inch for
2-3 HP engines, 3/4-inch for up to 8 HP or so, and one-inch for 10
HP and bigger. Averaged over time the larger but fewer exhaust
pulses of the big engine will make about the same flow rate as the
small, fast and frequent ones from the little engine.

In either case, the gas should flow out basically in the time of
an exhaust stroke. When you look at ratios of displacement and
speed, you can see that it works out pretty similarly for the flow
time to be in proportion to the degrees of crankshaft rotation for
both situations if the engines are similar in power.

Engine enthusiast Helen French lives in Leicester,
England.

Contact her via e-mail at: Helen@insulate.co.uk

You can join the Stationary Engine List on the Internet at:
www.atis.net

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